Circuit UI is SumUp's open-source design system for the web, a React component library with design tokens for color, spacing, and typography, plus Stylelint rules that enforce correct usage directly in the codebase.
Shipping custom Stylelint rules alongside the component library is a distinctive detail, it catches misuse of the design system (wrong spacing values, off-token colors) at lint time in CI, before a PR is even reviewed.
Developers building fintech or payments-adjacent web apps who want a real, installable React component library with built-in linting to enforce consistent usage.
It's genuinely open source on npm and GitHub, worth checking the Stylelint plugin specifically since it's a less common but genuinely useful enforcement mechanism.
The web implementation of SumUp's Circuit design system.
